A communication device such as a transmission device is generally provided with a monitoring device configured to detect a failure occurring inside the communication device and to output an alarm signal to the outside. Such a failure monitoring and outputting function by the monitoring device is generally called a housekeeping function or an office alarm function. Patent Document 1 discloses a monitoring method that automatically switches from a current digital communication line to a backup digital communication line upon receipt of an alarm indicative of communication line deterioration.    [Patent Document 1] Japanese Laid-open Patent Publication No. 03-007439.